Metal articles are ubiquitous in both civilian and military applications. For example, the U.S. military has used metal parts and objects for operational purposes since its inception. Tanks, ships, planes, armament, munitions and weaponry are several well-recognized platforms that include discrete parts, sub-assemblies, assemblies and system-level articles manufactured or formed from metal and metal-matrix materials. These articles typically have limited lifespan due to mechanical, electrical, chemical, and/or physical breakdown of one or more critical components. These limitations may, for instance, manifest as one or more of wear, corrosion, or surface finish degradation.
Standards and procedures exist for most manufacturing methods used in production of metal parts supplied to the military, however there is no MIL-STD, procedure, special process initiative, or engineering document that explicitly governs cryogenic processing or cryogenic treatment of metal parts for use by the US military. This absence of procedure has tended to severely limit the application of this technology, particularly in US military contexts.
Various techniques are known for cryogenic processing of materials to provide stress relief, enhance mechanical wear characteristics, reduce corrosion, and/or otherwise improve metallurgic, mechanical, electrical, and/or other properties of metal-matrix objects. Some of these techniques for and benefits of cryogenic processing are described, for example, in U.S. Pat. No. 3,891,477, issued June 1975 to Lance; U.S. Pat. No. 5,865,913, issued February 1999 to Paulin, et al.; U.S. Pat. No. 5,259,200, issued November 1993 to Karmody; U.S. Pat. No. 4,739,622, issued April 1988 to Smith; U.S. Pat. No. 5,174,122, issued December 1992 to Levine; U.S. Pat. No. 4,482,005, issued November 1984 to Voorhees; U.S. Pat. No. 7,744,707, issued June 2010 to Brunson; and U.S. Pat. No. 7,464,593, issued December 2008 to Masyada.
Various types of systems are also available for facilitating the cryogenic processing of metal-matrix materials. Some of these systems are described in certain of the references listed above. The systems typically facilitate a cryogenic treatment protocol. For example, the treatment protocol includes ramp down, cold soak, and ramp up operational steps, and the systems may allow for setting, controlling and monitoring temperature and time variables dictated by the various treatment protocols (e.g., by use of a thermostat, a programmable logic controller, a temperature recording device, etc.). Some treatment protocols further include one or more post-cryogenic treatment (e.g., tempering) phases, for example, to reduce embrittlement of the cryogenically treated material. The various systems may perform different steps of the protocol in different ways. For example, the cold soak stage may be implemented as a wet soak (i.e., a material undergoing treatment is fully or partially immersed in a cryogenic liquid) or a dry soak (i.e., a material undergoing treatment is fully or partially immersed in vapors of a cryogenic liquid).
The science behind the cryogenic effect is generally ascribed to changes within the crystalline lattice structure that occur following extended cold treatment, although the specific mechanism resulting in the cryogenic effect has yet to be fully explained. Observed under scanning electron microscopes, when material is exposed for a period of time at or near −300° F., primary and secondary carbides precipitate and nucleate within the material matrix and austenitic steel transforms into martensitic tetragonal structure without embrittlement, promoting stress relief, more uniform grain structure, increased densification and reduced crack initiation. Cryogenically treated material demonstrates significant reduction in abrasion and wear characteristics and corrosion effect and the process has been successfully used, for example, in tooling, engine components, gears, bearings and other industrial applications.
Processing protocols vary widely, for example, in relation to differences in material alloy, item weight, surface treatment, and matrix composition. These processing protocols are determined according to trial and error and limited empirical observations without guidance from any common or industry-recognized database that correlates a given processing protocol to a proven performance outcome. Further, there tends to be little industry oversight and, until recently, little relevant scientific data to support particular treatment methodologies. Accordingly, manufacturers or providers of cryogenic equipment and services have tended to differentiate themselves by closely guarding the time and temperature variables that form the content of a processing recipe. This has contributed to a general perception among the scientific community that the treatment process is questionable in effect and unreliable—in spite of industry-wide usage that argues and demonstrates via actual performance to the contrary.